The invention relates to an implantable cardioverter-defibrillator and/or cardiac pacemaker for stimulating the cardiac muscle (cardiac stimulation device), with at least one electrode for connection to the heart muscle, a programmable electrical circuit for producing series of stimulation impulses with impinge upon the electrode, and with an electronic memory connected to said circuit, with which over several periods of the heart beat the electrical phenomena inherent in the heart-beat, in particular systoles and their characteristic frequency f.sub.c, can be recorded and stored, the electrical circuit evaluating the stored electrical phenomena and emitting corresponding series of anti-tachycardiac and anti-bradycardiac stimulation impulses.
The two most important types of cardiac stimulation devices, cardioverter-defibrillators and so-called pacemakers, conventionally operate using types of stimulation which begin after the onset of cardiac tachyarrhythmias.
Cardiac stimulation devices of this type, in the form of cardioverter-defibrillators and or pacemakers with anti-tachycardiac or anti-bradycardiac modes of stimulation are known, for example according to U.S. Pat. No. 4,052,991 and U.S. Pat. No. 3,857,399. However, the stimulation devices according to the state of art emit stimulation impulses which, during anti-tachycardiac function, begin with a 1-s time delay after the onset of the tachycardia.
They operate in correspondence, that is, the period duration of the stimulation impulses is shorter than the duration of the systoles in tachycardia. This shortened impulses series is intended to "intercept" the extrasystoles, which become present in tachycardia. On perception of the systole frequencies arising during tachycardia, the delay time is followed by a continuous stimulation at a frequency f.sub.s, which is higher than the characteristic frequency, f.sub.e. This frequency f.sub.s is for the most part held constant.
The aim is to provide a cardiac-stimulation device, which is capable of preventing the occurrence of tachycardia at an early stage.
The aim is achieved by a cardiac-stimulation device, namely a cardioverter-defibrillator or a pacemaker of the type named in the preamble, characterized in that the electrical circuit is able, on detection of one or more of successive extrasystole(s) not of the characteristic frequency f.sub.e, to produce a series of stimulation impulses of frequency f.sub.g (t) which, after the appearance of extrasystole(s), begin after a programmable time interval (between 50 and 100% of t.sub.mean) at a frequency f.sub.gi higher than the detected previous frequency f.sub.g and are then steadily retarded, i.e. they steadily decrease in frequency f.sub.g until the frequency f.sub.g (t) essentially corresponds to the characteristic frequency f.sub.e, in particular that the frequency f.sub.g (t) falls below the characteristic frequency f.sub.g to a frequency f.sub.sub or ends with the frequency f.sub.c.
This type of production of a series of stimulation impulses is embodied in a cardiac stimulation device, which is itself known, as characterized in generic term in claim 1.
Following an extrasystole, the cardiac-stimulation device according to the invention produces an initially more rapid series of stimulation impulses at a higher frequency f.sub.gi than frequency f.sub.c (characteristic or natural frequency). A steady slowing then follows, until the frequency f*(t) corresponds to the characteristic frequency f.sub.c. The frequency f.sub.g preferably ends on attaining the characteristic frequency f.sub.c.
The cardiac-stimulation device according to the invention is therefore capable of producing, in addition to the known and still available modes of stimulation, another mode of stimulation setting in after each extrasystole, which is followed by a longer "compensatory" pause, without a prior delay of the order of 1 s for the first stimulation impulse, as in the state of art. It is therefore proposed that the time interval t.sub.i between the extrasystole and the first stimulation impulse is programmable with the frequency f.sub.gi. The time interval t.sub.i between the extrasystole and the first stimulation impulse preferably being, for example, between 60 and 90% of a mean time interval t.sub.mean between two systoles of characteristic frequency. The percentage can be variably programmed according to the prematurity of the extrasystole.
The frequency retardation may, for example, be such that the time between the stimulation impulses increases by at least 3% from one impulse to another, preferably by 6-18%.
To determine the frequency, a mean time interval t.sub.mean between two systoles of characteristic frequency is determined as a floating mean value over a plurality, for example 2-5, heartbeat periods.
On occurrence of two extrasystoles (ES) at a differing time interval, the frequency determination begins at the last systole corresponding to the characteristic frequency.
Retardation of the initially higher frequency may be linear, for example according to the equation: EQU f.sub.g (t)=f.sub.gi (1+t.multidot.(f.sub.e -f.sub.si)/t.sub.2)
where t.sub.2 is the endpoint of stimulation, when f.sub.gi is equal to f.sub.e or - in case of a missing characteristic frequency--the anti-bradycardiac rate of the pacemaker.
Retardation of the initially higher frequency may also be exponential, for example according to the equation: EQU f.sub.g (t)=f.sub.si .multidot.e.sup.alpha.t EQU where alpha=(1/t.sub.2).multidot.1n(f.sub.e /f.sub.gi)
Another useful property of the cardiac-stimulation device is that, on detection of a further extrasystole, the stimulation sequence currently operating can be discontinued and restarted after the extrasystole. This also ensures that characteristic normal systoles of the heart are not counterstimulated.
The aforesaid cardiac-stimulation device is further characterized in that it can be used to stimulate within the atrium and/or the ventricle of the heart.